The present invention relates to a ferromagnetic semiconductor of group IV, to its fabrication process, to an electronic component of the diode type for injecting spins into or collecting spins from another semiconductor respectively, or else of the type comprising an element sensitive to a magnetic field, and to uses of this semiconductor relating to this component. The invention applies in particular to the injection of a current of spin-polarized carriers into a semiconductor or to the collection of said current therefrom and to the measurement of magnetic fields via such a ferromagnetic semiconductor.
The injection of a current of spin-polarized carriers into a semiconductor, said current being characterized by an excess of one of the two populations of carriers present (for example the parallel-spin or spin-up population), has been the subject of several recent publications. For example, mention may be made, by way of explanation, of the electronic components described in the article by Datta and Das, Applied Physics Letters, 56, 665, 1990.
The application of this injection of a spin-polarized current is of great interest in microelectronics, but its development is thwarted through lack of suitable materials for forming the current injection electrode.
The reason for this is that although the usual ferromagnetic metals, such as iron and many of its alloys, do have some of the required qualities, such as a high spin polarization and ferromagnetic behavior at room temperature, their electrical resistance is several orders of magnitude different from that of semiconductors, thereby causing great difficulties of implementation and requiring the current to be injected by a tunnel effect. This has the drawback of requiring the growth of a hybrid heterostructure of the semiconductor/tunnel barrier/ferromagnetic metal type, such a heterostructure being difficult to produce.
In contrast, there are semiconductors referred to as diluted magnetic semiconductors (DMSs) which do not have this drawback of having a very different resistivity from that of ordinary semiconductors. These DMSs typically consist of a semiconductor matrix of groups III-V, IV or II-VI in which magnetic impurities such as manganese, iron, chromium, cobalt or nickel are diluted.
In the case of a manganese dilution, manganese being an acceptor in III-V or IV semiconductors, the charge carriers consist of holes. When the manganese concentration and the density of holes (created naturally by the presence of manganese or intentionally introduced by co-doping) are sufficiently high in the DMSs, the latter may become ferromagnetic and the exchange coupling between manganese ions is induced by the holes.
A major drawback of these DMSs is that they all have at the present time a Curie temperature TC (the temperature up to which the semiconductor exhibits ferromagnetic properties) at or below room temperature (typically ≦300 K approximately). For example, the reader may refer to the article by K. W. Edmonds et al., Phys. Rev. Lett. 92, 037201, 2004, which describes a semiconductor of formula GaMnAs having a Curie temperature of about only 159 K, and to the article by H. Saito et al., Phys. Rev. Lett. 90, 207202, 2003, which describes DMSs satisfying the formula Zn1-xCrxTe and having a Curie temperature approximately equal to 300 K (±10 K), when x=0.20.
Another drawback of these DMSs lies in the undesirable but frequent formation of small ferromagnetic metallic precipitates within the semiconductor matrix, this being inconducive to genuinely ferromagnetic properties in the case of these DMSs and making the step of growing the crystals very difficult to carry out.
It should also be noted that the use of these gallium-based or tellurium-based materials is very difficult to envisage on silicon substrates, silicon being the base material of the microelectronics industry.
U.S. Pat. No. 6,946,301 discloses a thermal evaporation process for fabricating a ferromagnetic semiconductor of the GeMn type that has a Curie temperature possibly up to 250 K, for a manganese content of about 35%.
U.S. Pat. No. 6,307,241 teaches, in its single exemplary embodiment, how to fabricate a ferromagnetic semiconductor of the III-V (GaAs) type with a Curie temperature TC above 400 K using the ion implantation technique, with the implantation of manganese ions (Mn+), followed by an annealing operation. As known to those skilled in the art (see in particular the article “Magnetooptical Study of Mn ions Implanted in Ge” by Franco D'Orazio et al, IEEE Transactions on Magnetics, Vol. 38, No. 5, September 2002), it should be noted that this implantation technique is not suitable for fabricating ferromagnetic semiconductors of group IV (typically based on germanium) with TC≧350 K, it being specified that the phase thus obtained, of the Ge3Mn5 type, has a TC never exceeding 300 K.
A major drawback of these known magnetic semiconductors, of the diluted or even ferromagnetic type, lies in their relatively low Curie temperature, which is generally limited to about 300 K. In addition, when the measured Curie temperature is close to 300 K, it is difficult to exclude the presence of the Ge3Mn5 metallic phase, the Curie temperature of which is specifically close to 300 K.